1. Field of the Invention
The present invention relates to methods and devices for optical communication.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Optical communication technology offers the promise of data rates that are significantly higher than those provided by conventional radio-frequency-based technology. For deep-space applications, power efficient communications is possible in part due to the large effective power gain from narrow optical beamwidths. Consequently, a necessary component of optical communication systems is highly accurate and stable laser beam pointing.
In order to establish and maintain an optical link, accurate uplink and downlink pointing must be performed in the presence of spacecraft motion and disturbances. Disturbance suppression can be achieved through a combination of passive isolation to reduce mechanical coupling between the spacecraft and flight terminal platform, and active cancellation of pointing errors through platform steering and downlink beam steering via a fine steering mirror. Local reference sensors such as inertial reference units may be utilized to provide highly accurate information for active disturbance cancellation, but increase mass and power on the flight terminal, and do not provide a pointing reference to the Earth. In order to minimize mass on the spacecraft transceiver, the Deep Space Optical Communication (DSOC) project at the Jet Propulsion Laboratory uses an uplink beacon transmitted from the ground terminal to provide a reference spot position that may be tracked by the flight terminal. 1 By estimating the uplink signal position, the flight terminal platform attitude may be adjusted and the point-ahead angle for downlink transmission may be calculated and implemented. Furthermore, an uplink beacon can also carry command and configuration data. By using a single photodetector array for both pointing and communications, rather than a more conventional architecture consisting of separate detectors for tracking and communications, beam alignment errors and optical losses as well as overall system complexity may be minimized. A photon counting array possesses the best combination of sensitivity and bandwidth for these purposes. Signal processing algorithms for uplink spatial acquisition and tracking, parameter estimation, and command telemetry processing must be therefore be developed in order to simultaneously support tracking and communications for the deep-space optical link.